Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy.
In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
2. Image-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element.
3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an image-wise heated donor element onto a receptor element.
Most of the "direct" thermographic materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced. A wide variety of chemical systems has been suggested some examples of which have been given on page 138 of "Imaging Systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), describing the production of a silver metal image by means of a thermally induced oxidation-reduction reaction of a silver soap with a reducing agent.
JP 57-69,095 discloses a support coated with a subbing layer containing an ionomer, and then coated with a heat-sensitive recording layer whose solubility in water decreases upon heating to give a thermographic sheet. EP-A 496 017 discloses a heat-sensitive recording medium, comprising: (a) a synthetic-resin support; (b) an ionomer-resin layer on said support; and (c) a heat-sensitive recording layer on said ionomer resin layer.
GB 2 114 767 discloses a thermosensitive recording sheet comprising: a support material; a primer layer formed on the support material and comprising a filler and a binder agent; a thermosensitive colouring layer formed on the primer layer and comprising a colourless light-coloured leuco dye, and an acidic material which colours the leuco dye upon application of heat hereto; and a protective layer formed on the thermosensitive colouring layer and comprising a water-soluble agent and a filler.
U.S. Pat. No. 5,006,451 discloses in the specification that "polymeric subbing layers used to promote the adhesion of coating compositions to polyester film supports are very well known in the photographic art. Useful compositions for this purpose include interpolymers of vinylidene chloride such as vinylidene chloride/acrylonitrile/acrylic acid-terpolymers or vinylidene chloride/methyl acrylate/itaconic acid-terpolymers".
However, the inventors of the present invention found that the light-stability and archivability of substantially light-insensitive thermographic materials coated from aqueous media were surprisingly, considering their thinness relative to the thickness of the thermosensitive element thereof, dependent upon the choice of subbing layer and that, moreover, polymer subbing layers used to promote adhesion between polyester support and conventional photographic emulsion layers were in the main unusable (see COMPARATIVE EXAMPLE 2) due to prohibitive light-sensitivity and poor archivability. There is therefore a need for subbing layers for use on the supports of thermographic materials, which have no adverse effect upon the light-stability and archivability of thermographic materials.